cifs: do not use uninitialized data in the owner/group sid

When idsfromsid is used we create a special SID for owner/group.
This structure must be initialized or else the first 5 bytes
of the Authority field of the SID will contain uninitialized data
and thus not be a valid SID.

Signed-off-by: Ronnie Sahlberg <lsahlber@redhat.com>
Signed-off-by: Steve French <stfrench@microsoft.com>